VHF Directional Finder Loop Aerial

7/26/2019 VHF Directional Finder Loop Aerial

1/4

F

C JUDD G2BCX

in

radio direction finding

in map reading as well, is provided by v.h.f. OF hunts,

horizontally

or dipole could be used to obtain

is not defined enough for

it is properly

is better, as the nulls

in

the polar pattern

If

the hidden

is using a vertically polarised aerial, then a

is of no use, as it becomes omnidirectional when

in that the maximum signal indication

not defined

well

enough to obtain accurate bearings.

ZL

Special PW, May 77) is an exception

is very useful for OF work as

it

has one

is cardioid when the

is operated vertically, but more of this later.

b)

The loop aerial described in this article is for use when

signals are from a vertically polarised source. It consists

of a split loop, one half-wavelength

in

circumference, tuned

and balanced to operate with 50 ohm coaxial cable as

shown

in

Fig. l(a). The current and voltage distribution,

together with the direction of the current in each half-loop

(arrows) are shown

in

Fig. l(b). Since the currents

in

each

half are in phase opposition, the polar pattern is a figure-

of-eight, like that of a horizontal dipole, with two maxima

and two minima as in Fig. 1 c).

This loop will also operate very efficiently as a trans-

mitting aerial with a

V.S.W.r.

close to 1 : 1 when properly

tuned. This means that stations, mobile or handheld,

taking part in the OF hunt can communicate with each

other to pass bearings and map readings, etc., without

having to change aerials. It

is

appreciated that this loop

system does not provide a means of sensing, i e of

determining which minimum is giving true direction, but

this is not greatly important when two or more stations

at reasonable distances apart are working as a team.

Information on direction finding is given later.

Making the loop erial

General construction details for the loop are given

in

Fig. 2 The loop itself is made from 4 7mm -ft in) diameter

aluminium rod although

63mm

tin) may be used.

Bending the rod to form the two half-circles

is

not difficult

if done gradually and with aid of gentle heating from a

_small blow-lamp. The bottom ends are flattened by

hammering on a hard surface, after which they can be

filed smooth and drilled 4BA clearance as shown.

The lower mounting block, and the top block that

supports the upper ends and keeps them separated, must

be made from Perspex or similar good-quality insulating

material. The upper ends are at high r.f. potential. The

box for mounting the loop on and housing the two 30pF

trimmers may

be

a plastics electrical junction box, or

similar, with removable lid and

of

dimensions approxi-

mately as shown. The handle can be a short length of

25mm dowel or broomstick, secured to the underside of

the box with a wooden collar, by gluing and screwing.

The diagram Fig. 3 shows how provision is made for

a compass platform and a sighting bar. Note also the hole

each side of the box allowing access to the

30pF

trimmers

after the aerial is assembled and ready for tuning.

Connection to the loop from a length of 50 ohm coaxial

cable is made via the S0239 socket at the base of the box.

Max

Max

c)

Fig. 1 a): Configuration

of

the

tA-loop

Fig. 1 b): Current and voltage

distribution

Fig. 1 c): Polar pattern of the

loop

Practical Wireless October 1979


2/4

Self tap

5BA

38

1

1 2

1 1

I

I

I I I

I

-t 4 BA Clear

1 L:: K

I I I 50 I I I 10mm thick

lockdrilling detaill

I Sighting

bar

block

4BA

I

I

I

I

_

1

I

I

Plastics box

I

100x100x50 opprox

see text

Fig

:

General constructional details

All dimensions in mm


51


3/4

80

plywood

I I

I ILoop

I I

I I

r - - - - - - - - 11

-----------

: Compass :

25

5

Sighting bar 80

Aluminium)

16swg

adjustment -1---- I

30 pF

trim

mers

-------- ,----- , -r i . . . , -- , ------ ,------

dimensions in

Fig 3: Arrangement for a compass platform

and

hecking

the

loop

The length of the external coaxial cable will depend on

For example, when connected

a mobile rig, one will need to use the loop a metre or

from the car to avoid reflection from the car body,

that

about

3 metres of cable should be used. Handheld

operators will need only about 1 metre. The

is otherwise not critical and

may

be chosen entirely

suit convenience.

With the loop aerial connected

to

the

transmitter/

of coaxial cable decided upon,

that

is necessary is

to adjust the two

trimmers

for

power (on transmit) into the aerial, or better

V.S.W.r.

00 this

at

midband

(l45MHz)

and

good to the

outer

limits

of

the band. If

Fig 4: Actual

polar pattern

of prototype

loop aerial

a receiver only is to be used, then adjust the trimmers

until a received signal is

at

maximum. Remember the loop

has two nulls and

two

maxima,

so

for a

transmitting

test

with another station the loop must be end-on to that

station for maximum signal. Checks for the two nulls

(minimum signal) should be

made out

of

doors

and well

clear of other 201 aerials, and indeed any other conducting

elements likely to

cause

reflection and give rise to false

nulls. Each null should be

sharp

and reduce a received

signal down to noise level

or

even completely out. Check

also that the nulls are 180

0

apart.

A small

hand

type compass can now be fitted to the

compass

platform.

t

need

not

be

permanently

fixed,

but

. instead secured with strong elastic bands and aligned so

that

0/

360

0

points to

one

side of the loop, i.e. in the

direction of the null

in

line with the sighting bar.

Direction

Finding

The polar pattern shown in Fig. 4, taken with the

writer s Polar Pattern Indicator aerial performance display,

is from the prototype loop. It clearly shows a virtually

perfect figure-of-eight shape with two well-defined nulls at

305

0

and

125

0

respectively

and

therefore 180

0

apart. The

other

pattern (Fig. 5) is

an almost

perfect cardioid from

a two

e

lement ZL Special. In

team

operation

in

a

OF

hunt this can be used for

sense

of direction. Once this

has been

obtained,

the

remainder

of the team

can

be

notified that the hidden transmitter is to the

North or

the

East (or whatever) generai direction from them.

For

those new to radio direction finding the following

notes may be found useful.

Ordnance

Survey maps

1 : 50000 series with 1 kilometre squares

are

ideal for

OF

work. Indeed, they are essential if

accurate

results are to

be obtained, especially when the start

of

operations may

be several miles from the transmitter to be located. A

team of three

is

ideal if location

is

to be found quickly.

although two

operators

could work together fairly effi-

ciently. A lone

operator

would need

to move about

a lot

more in

order

to get the necessary cross bearings.

The illustration (Fig. 6) is based on the 1km squares

of

a I :

50000

Ordnance Survey map, and here the hidden

transmitter is on a reference of approx 67 Easting and 92

Northing (note that on a full-scale map, the Eastings and

Northings are each given in three figures, making six

Fig

5:

Cardioid

pattern

of the 2 element Z L

Special



4/4

12

11

1

9

8

7

12km 6

5

4

3

2

o

Grid refelience map

Real

l

squares 1km) /

1

I

Transmitter

-

.

..-::::

p:;..

/;

.

A

/. .

/

//

I

,

352Mag

gives 002grid

024 Mag

gives 034 grid

/

. I

lReal

y

/ .

. .- -

/

-

oives

7

- Cocked Hat

North

\

T

Grid

... 1

North

C

0 1

2 3 4 5 6 7

8

9 1

10km

rrors

in above example

f

Ixes:-

A

approx 4

B a pp

rox

3

C approx 2

I

I

For bearings

related

to grid north add 10

to compass bearings

grid

Fig.

6:

Example of obtaining a fix on a hidden trans-

mitter from three positions. Errors shown are about

average in practice but do produce the cocked hat

area enclosing

the

true position

figures for the full grid reference). The bearings taken by

DF

stations A, Band C are shown by the solid lines and

are about average as as error is concerned. They form

a triangle

of

cocked hat , around the true position

of

the

transmitter. Remember that we have started from several

miles away, but at this stage have located the transmitter

to within about a square kilometre. The next step is for

each member

of

the team to move

in

closer,

in

this case

A goes to point 96, B to 86 and C to 88. Even with

similar errors in bearing, e.g., between 2 and 4 degrees,

the now much shorter range should establish a location

of the transmitter to within an area of about Olkm or a

little over 100 yards square.

The

procedure can be repeated

and again, with average errors, location to within OOlkm

or about 1 yards square should be possible.

Remember,

that

an

Ordnance

Survey map grid North

is

slightly different from true North, and magnetic (com-

pass) North is different from true.

The

total amount to be

added to a magnetic bearing for the grid map

is

about

1

0

If, for example, your magnetic bearing is 024

0

then

using the vertical grid lines

of

the map as 0/360

0

(North)

the bearing to be plotted on the map will be 034

0

Ascertaining the null

is

usually best done by listening,

preferably on headphones, and accuracy

in

obtaining the

minimum stems largely from practice in aurally estimating

this, although a signal level meter is helpful of course.

Remember that nulls can sometimes be confused by the

arrival of signals from other than the direct path, due to

. reflection, so if a null appears to be confused or

not well

defined, try moving to another position.


So You Want to Pass the RAE?

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of

the complete series, including details

of

the

new examination format being introduced in 1979,

is

now

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VHF Directional Finder Loop Aerial

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